gptimer_get_captured_count() function

- ESP_OK: Get GPTimer captured count value successfully - ESP_ERR_INVALID_ARG: Get GPTimer captured count value failed because of invalid argument - ESP_FAIL: Get GPTimer captured count value failed because of other error

Different from `gptimer_get_raw_count`, this function won't trigger a software capture event. It just returns the last captured count value. It's especially useful when the capture has already been triggered by an external event and you want to read the captured value. This function is allowed to run within ISR context If `CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM` is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.